In practice, this type of panel integrates a cellular core, such as a honeycomb structure flanked on the incident sound wave side, with an acoustic damping layer and, on the opposite side, with a rear reflector.
The acoustic damping layer is a porous structure with a dissipating function, which is to say partially transforming the acoustic energy of the sound wave passing through it, into heat.
This porous structure can be for example a metallic cloth or a cloth of carbon fibers whose weave permits fulfilling its dissipating function.
As these acoustic panels should, for example in the case of panels for the nacelles of jet engines, also have sufficient structure properties particularly to receive and transfer aerodynamic and inertial forces and forces connected to the maintenance of the nacelle, toward the structural nacelle/motor connections, it is necessary to give the acoustic damping layer structural properties.
To this end, it has already been proposed to provide an acoustic damping layer with two superposed components, one structural and the other porous and dissipating, the structural component being either disposed between the cellular structure and the dissipating component, as shown by the patent GB 2 130 963, or disposed in contact with the incident sound wave, as shown by the document EP 0 911 803.
The invention envisages more precisely panels of this latter type, which is to say comprising a resisting layer with a structural component turned toward the incident sound wave, but is applicable also to panels whose resistive layer comprises a structural component interposed between the dissipating component and the cellular structure.
The structure of the panel according to EP 0 911 803 has the drawback of a resistive layer formed by two metallic superposed layers, namely a cloth and a sheet. The metal used to produce the metallic cloth is preferably stainless steel, whilst the structural layer is an aluminum sheet. In addition to the fact that the metal-metal securement requires a particular technique which is not entirely satisfactory, the use of the two metals of different structure induces corrosion by the appearance of a galvanic couple. Moreover, the density, although low, of the metals used increases substantially the weight of the acoustic panel.
The use of composite materials to produce such dissipating or structural layers is well known and permits providing an acoustic panel that is lighter than an acoustic panel using metal whilst maintaining for said panel its structural and acoustic characteristics.
There exists an abundant literature describing acoustic attenuation panels of the sandwich type comprising an acoustically resistive layer formed by a pierced non-metallic sheet used alone or in association with a porous layer. However, these sheets are generally constituted of plastic materials with high strength at elevated temperature or of plastic materials reinforced with fibers, particularly graphite.
Moreover, these sheets, metallic or non-metallic, merging structural and acoustic characteristics, all comprise circular perforations, aligned or substantially along a diagonal.
To maintain a quantity of open surface permitting good acoustic damping, it is necessary to perforate the structural layer with a suitable number of openings. As a result, this layer is rendered fragile, on the one hand, by the removal of material onto which it is subjected and, on the other hand, by the arrangement of the openings. Thus, the remaining material between two openings does not permit the structural layer to support the transfer of mechanical, aerodynamic and inertial forces toward the motor frame. So as to overcome this problem, it is thus necessary to reinforce said layer by increasing its thickness or decreasing said quantity of open surface, which is at the cost of the acoustical damping quality of said panel.
On the other hand, in the case of an arrangement of the perforation openings on the diagonal, the use of composite materials such as a layer of carbon is not suitable. Thus, the fibers of said material are broken by the removal of the material and their discontinuity does not permit the transfer of forces mentioned above. For this reason, it is necessary to increase the thickness of said structural layer, to the detriment of its weight.
Moreover, the shape of the openings, their symmetrical distribution in the structural layers of the above type, give to them an isotropic mechanical strength which does not in any way take account of the distribution of forces which are to be resisted by the acoustic panel. The forces being greater in the longitudinal direction than in the radial direction, it is thus necessary to produce a panel having a thickness suitable for the transfer of longitudinal forces but over-dimensioned for the transfer of radial forces.